.container {
  max-width: 500px;
  margin: 0 auto;
}
.bigbox {
  padding: 0.5vw;
  display: grid;
  grid-gap: 0.5vw;
  grid-template-columns: repeat(3, 1fr);
  justify-content: center;
}
.smallbox {
  width: 100%; /* 不明白为啥宽得设100％，高不用。*/
  height: 0;
  padding-bottom: 100%;
  background-color: orange;
  border-radius: 5%;
}
.btn {
  display: grid;
  justify-content: center;
  grid-template-columns: 1fr;
}
.btn > button {
  padding: 0.2vw;
  margin: 0.5vw;
  height: 2em;
  border: 2px solid orange;
  background-color: white;
  font-size: 1em;
}
button:hover {
  background-color: orange;
}
